In recent years, a TFET (Tunnel Field-Effect Transistor) that employs a quantum-mechanical effect of electrons has been developed. In this TFET, by applying a voltage to a gate electrode, BTBT (Band To Band Tunneling) occurs between a source and a channel. As a result, the TFET becomes an on-state.
Generally, a TFET has a BTBT threshold voltage that depends on the impurity concentration of a diffusion layer and corresponds to the respective impurity concentrations. That is, a tunneling current that is generated from a part of a source diffusion layer having a low impurity concentration flows at a small gate voltage (a low threshold voltage). On the other hand, a tunneling current that is generated from a part of a source diffusion layer having a high impurity concentration flows at a gate voltage (a high threshold voltage) applied at a level higher than the small gate voltage. Because an actual source diffusion layer of a TFET has an impurity concentration gradient, when the voltage of a gate electrode is swept to an on-side, BTBT having various threshold voltages occurs in the TFET one after another. Accordingly, a current that is observed when the TFET performs switching has an envelope of a tunneling current that corresponds to the respective impurity concentrations. In such a TFET, sub-threshold characteristics (hereinafter, also “SS characteristics”) on a low-voltage side are originated from a tunneling current when a threshold voltage is small, that is, a tunneling current that is generated from a part of a source diffusion layer having a low impurity concentration. In BTBT that occurs in the source diffusion layer having a low impurity concentration, an electron tunneling distance is very long. For this reason, the SS characteristics are very moderate. Therefore, according to a conventional TFET, because a tunneling current component in a low gate voltage area is dominant, it is very difficult to obtain steep (sharp) SS characteristics.